Analysis of fluid flow and heat transfer in a double pipe heat exchanger with porous structures N. Targui, H. Kahalerras * Faculty of Mechanical and Process Engineering, Houari Boumediene University of Sciences and Technology, B.P. 32, El Alia, Bab Ezzouar, 16111 Algiers, Algeria article info Article history: Received 15 November 2006 Received in revised form 3 July 2007 Accepted 25 February 2008 Available online 8 April 2008 Keywords: Double pipe heat exchanger Porous structures Heat transfer enhancement abstract A numerical study of flow and heat transfer characteristics is made in a double pipe heat exchanger with porous structures inserted in the annular gap in two configurations: on the inner cylinder (A) and on both the cylinders in a staggered fashion (B). The flow field in the porous regions is modelled by the Darcy– Brinkman–Forchheimer model and the finite volume method is used to solve the governing equations. The effects of several parameters such as Darcy number, porous structures thickness and spacing and thermal conductivity ratio are considered in order to look for the most appropriate properties of the por- ous structures that allow optimal heat transfer enhancement. It is found that the highest heat transfer rates are obtained when the porous structures are attached in configuration B especially at small spacing and high thicknesses. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction There are various techniques [1] to increase the heat transfer in tubular heat exchangers such as treated surfaces, rough surfaces, extended surfaces, surface vibration, fluid vibration, etc. A lot of re- ports for numerical and experimental simulations for laminar or turbulent flow and heat transfer, concerning the employment of solid extended surfaces such as fins and baffles, can be found in the literature [2–12]. Most of these works discussed the optimal spacing, shapes, and orientations of these structures that enhance heat transfer performance for a given pumping power or flow rate. In this passive technique, the attachments provide an additional heat surface area and promote turbulence. In the other hand, sev- eral studies have shown that the heat transfer enhancement in tubular heat exchangers can also be achieved by use of porous materials. In this context, Chikh et al. [13–15], Bouhadef et al. [16] and Alkam and Al-Nimr [17] investigated the thermal perfor- mance of a double pipe heat exchanger in which a porous substrate is inserted. They found that porous medium may be used for insu- lation or enhancement of heat transfer according to its physical properties. Guo et al. [18] showed that a significant augmentation of heat transfer and a reduction of pressure drop may be achieved in an annulus by use of porous medium and pulsating flow. Allou- ache and Chikh [19] presented a thermodynamic analysis in a dou- ble pipe heat exchanger with a porous layer inserted in the annular gap in order to reduce the rate of entropy generation. The use of porous extended surfaces such as fins and baffles to augment heat transfer rate seems to be more interesting than the conventional solid ones due to their larger effective surface area and the lower pressure drop generated in comparison to the latter ones. Some numerical and experimental investigations have been recently performed in channels fitted with porous structures. Hwang [20] conducted experiments to study the fluid flow and heat transfer in a channel where porous baffles were mounted on the top and bottom walls in a staggered manner. This work showed that the porous baffled channel have a significantly lower friction factor. Kiwan and Al-Nimr [21] studied numerically the thermal performance of porous fins attached on a hot surface. They found that using porous fins may enhance heat transfer in comparison to solid ones and as result save fin material. An experimental study was conducted by Ko and Anand [22] in a staggered porous baffled channel. Experiments showed that the heat transfer enhancement per unit increase in pumping power was less than one for the range of parameters studied in this work. Yang and Hwang [23] pre- sented a numerical study of turbulent flow and heat transfer in rectangular channel with porous baffles. Analysis was also done for solid baffles for the sake of comparison. A similar study, but for a laminar flow, was conducted by Monte Da Silva Miranda and Anand [24] in a two-dimensional parallel-plate channel whose walls were subjected to uniform heat flux heating condition. The above literature review revealed that the most undertaken works, in heat exchangers, were devoted to the problems concern- ing the employment of solid extended surfaces or porous layers. The use of porous fins and baffles have been limited in channel configuration and, to the best knowledge of the authors, this latter 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.02.010 * Corresponding author. Tel.: +213 772 987 151. E-mail address: kahalerrashenda@yahoo.fr (H. Kahalerras). Energy Conversion and Management 49 (2008) 3217–3229 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman